1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image and an image forming apparatus such as a multifunction peripheral, a copier, a printer, or a facsimile machine which includes a fixing device.
2. Description of Related Art
In general, an image forming apparatus such as a printer of an electrophotographic system includes a fixing device that includes a heater, heats/pressurizes a toner image formed on a sheet, and fixes the toner image to the sheet. Then, a member for heating toner, such as a roller, a belt, or a film, is heated by the heater. However, for example, there is a problem that toner is not fully fused if a temperature of the roller or the like is too low and that the toner is excessively fused if the temperature is too high. Therefore, the temperature of the roller or the like needs to be maintained at a temperature suitable for fixation. To maintain the temperature of the roller or the like at the suitable temperature, it is necessary to sense the temperature of the roller or the like. To this end, for example, there is known a fixing device that senses the temperature inside the fixing device.
Specifically, for example, disclosed is a heating device (fixing device) including: a heating body; a pressure member that forms a press-contacting nip portion in contact with the heating body; and at least two temperature sensor elements that sense a temperature of the heating body, in which: after the pressure member is caused to pivot and a heating process is performed for a material to be heated that has been conveyed/introduced into the press-contacting nip portion, the material to be heated is discharged, the heating device is caused to stop; and if the material to be heated having a width shorter than a predetermined size is conveyed, at least one of the temperature sensor elements is located in a position to be a sheet passing portion, at least another one of the temperature sensor elements is located in a position to be a non-sheet passing portion, and a time from the end of the heating process for the material to be heated until the stopping of the driving of the heating device is caused to change according to a difference between temperatures sensed by the temperature sensor element at the non-sheet passing portion and the temperature sensor element at the sheet passing portion. With this structure, for example, needless driving of the heating device after the end of the heating process may be prevented, which elongates the life of the heating device.
As in the above-mentioned heating device, the fixing device may be provided with a plurality of temperature sensor units. In addition, the temperature sensor unit includes a temperature sensing element (for example, thermistor), and performs temperature sensing by bringing the temperature sensing element into contact with an object of sensing such as the roller.
Meanwhile, in a case of a fixing device in which a sheet is conveyed with a center part of an axis direction of a roller or the like aligned with a center of the sheet (hereinafter, referred to as “center sheet passing”), the sheet is occasionally conveyed with the sheet positioned close to one side of the roller or the like due to an uneven pressing force at a nip or other such factor. The one-sided conveyance of the sheet is likely to cause a paper jam of the sheet, and the temperature may keep rising without heat loss particularly at an end portion through which the sheet does not pass, which necessitates replacement of the fixing device. To sense such one-sided conveyance of the sheet, for example, the temperature sensor unit is provided to each of two portions (for example, both end portions) of the roller or the like in its axis direction. Then, during printing, when a difference between the temperatures sensed by using both the temperature sensor units becomes equal to or higher than a fixed value, it is sensed that the one-sided conveyance of the sheet is being performed.
Meanwhile, the object of sensing such as the roller is caused to rotate due to the sheet conveyance or the like. To protect against friction due to rotation or for other such purpose, the temperature sensing element in contact with the roller or the like may be covered with a protective member such as an insulating tape. However, the rotation of the object of sensing such as the roller may cause the protective member to peel off while the image forming apparatus is being used.
Then, the temperature sensing element from which the protective member has peeled off comes into direct contact with the object of sensing such as the roller. In other words, in comparison with the temperature sensing element from which the protective member has not peeled off, a distance from the roller or the like becomes smaller (the distance from the roller or the like becomes almost zero). This may cause a difference between the sensed temperature obtained by using the temperature sensing element from which the protective member has peeled off and the sensed temperature obtained by using the temperature sensing element from which the protective member has not peeled off. Note that the peeling off of the protective member is not so serious an issue that necessitates the replacement of the fixing device and is not so harmful as one-sided sheet conveyance, and hence correction of the sensed temperature suffices.
However, if there is a difference between the temperatures sensed by the temperature sensor units, a problem arises in that it may be erroneously sensed that the one-sided sheet conveyance is being performed even if the difference is ascribable to the peeling off of the protective member. Accordingly, there arises another problem in that it is necessary to judge whether or not the temperature difference is caused by the peeling off of the protective member.